This invention relates to the manufacture of miniature tubular radioactive light sources and, more particularly, to a method and apparatus for subdividing a long phosphor-coated tube filled with a radioactive gas into a plurality of individual sealed segments.
Self-illuminating devices are known which utilize a glass tube coated on the inside with a phosphor and filled with a radioactive gas such as tritium or Krypton 85. In the manufacture of miniature light sources of this self-illuminating type, it has been the practice to provide a long tube which is coated inside with a luminophor, evacuated, filled with the radioactive gas, and sealed at the ends. The tube is then subdivided into segments by melting the glass, and fusing the glass in the process to seal the ends of the segments. To help form the sealed ends of the tube segments and prevent escape of the radioactive gas, a pressure higher than the gas in the tube is maintained outside the tube. When the glass of the tube melts, it collapses to form the sealed ends. Laser beams have been used to heat the glass tube in the subdividing process. U.S. Pat. Nos. 3,706,543 and 3,817,733 describe arrangements incorporating laser beams for subdividing radioactive gas-filled tubes.
Various problems have been encountered in applying the techniques described in the prior art. In the prior art, the practice has been to hold the tube in two spaced chucks which rotate the tube in the laser beam so that the beam effectively scans the whole perimeter of the tube during the melting process. In practice, it has been found difficult to rotate the two chucks exactly in unison while the cut is completed. Any slight deviation from perfectly uniform rotation tends to stress the tube, i.e., twist it or bend it, which may cause fractures. The larger the cross-sectional dimensions of the tube, the more difficulty is experienced in maintaining perfect alignment and rotation of the tube during the cutting process.
Another problem encountered is that the tube is not easily cut at more than one point at a time, since each segment must be held and rotated in exact alignment with all the other segments during the cutting process so that no stress is placed on the tube. As a result, it has been the practice to only cut one segment at a time, advancing the tube between cuts to provide successive cuts along the length of the tube. As each cut take place, the total volume of the radioactive gas is reduced and therefore the pressure is increased. As each segment is cut off, the pressure in the remaining portion of the tube increases by an incremental amount. Thus, there is a difference in gas pressure between the first segment cut from the tube and the last segment cut from the tube. This results in a nonuniformity of brightness in the successive tube segments. This increase in pressure also requires an adjustment of the outside pressure with each successive cut to maintain the pressure differential necessary to achieve proper collapse and fusion of the tube ends as each new segment is cut. Summary of the Invention
A feature of the invention is the modulation of the velocity and/or intensity of the laser beam. In the case of back and forth beam scanning across at least one side of the tube, the velocity of the beam is smaller at the extremities of the beam path than at the middle of the beam path. This transfers more heat to the ends of the tube where there is more glass to be heated. The intensity of the laser beam is also increased during the subdivision of a segment to heat the cut line uniformly to a temperature below the fusion temperature of the tube before division of the tube begins.
Another feature of the invention is a tube holder having a slot in which the long tube slidably fits during its subdivision. The long tube is held in the slot on both sides of the cut line by means that permit the tube to be pushed through the slot by a tube advancing mechanism after each cut. For tubes with a rectangular cross section, the slot preferably has a dove tail cross section.
Another feature of the invention is defocusing the laser beam after the tube divides to anneal and remove discoloration from the divided ends while still in a tube holder, which holds the tube on both sides of the division. This, in effect, eliminates the requirement for a separate annealing and discoloration removal step.
Another feature of the invention is a plurality of focused laser beams simultaneously directed onto a long, sealed, radioactive gas-filled glass tube having a longitudinal axis at spaced intervals along the longitudinal axis. Relative movement is introduced between the laser beams and the tube transverse to the longitudinal axis of the tube to heat and soften the tube along a plurality of cut lines at spaced intervals simultaneously until the tube divides along each cut line to form ends and collapses to seal the divided ends. By thus simultaneously subdividing the long tube into a number of individual sealed segments, such segments are at substantially equal pressure and, therefore, exhibit substantially uniform brightness, the pressure outside the tube does not have to be readjusted during the subdividing process, and less time is required to carry out the process. Furthermore, the beam from a single laser can be split into the plurality of laser beams required for such simultaneous subdivision.